The present invention relates in general to scanning radiography with beam equalization and more specifically to a stationary equalization detector including electronic scanning or including variable detector channel size.
Scanning or slit radiography has been known for a number of years as a technique for reducing x-ray scatter in the radiographic image. In the known scanning radiographic systems, a moving x-ray beam is produced by a movable x-ray source to which is attached single or multiple collimators or slits thus producing a moving x-ray beam. Alternatively, the x-ray source may be stationary while the collimator is movable to create the moving x-ray beam. Known scanning x-ray systems include a movable pencil beam which scans the object in a two-dimensional raster scan and a fan beam approach which scans the object in a single direction. The radiographic image produced by either of these two methods is being used for medical purposes.
It frequently occurs that the body to be imaged includes both material that has high x-ray attenuation and material that has low x-ray attenuation. In order to avoid an x-ray image of such body in which some parts of the image are overexposed while other parts are underexposed, dynamic equalization of localized x-ray exposure has been employed. As a result, the dynamic range of the image is compressed to be within the dynamic range of the film or other detector which forms the image.
Scanning equalization is usually achieved with a separate equalization detector (i.e., separate from the image detector) which often scans along with the x-ray beam behind the body being imaged. The output of the equalization detector controls the x-ray exposure to each portion of the body, usually by varying an amount of attenuation introduced between the x-ray source and each respective body portion. Alternatively, the exposure time or the x-ray intensity can be varied, as in the scanning pencil beam approach. Thus, the image detector receives an exposure which is controlled to be within its dynamic range.
An example of the prior art using a scanned fan beam is U.S. Pat. No. 4,715,056 issued to Vlasbloem et al. on Dec. 22, 1987. This radiographic system employs a slit diaphragm which moves relative to an x-ray tube. An x-ray detector or a scintillator for producing an image moves along with the scanned fan beam to receive x-rays after they have passed through the body being imaged. Light from the scintillator is projected onto a film which records the image. An additional light detector for controlling equalization scans along with the image detector (e.g., scintillator) to sense the image intensity for a plurality of image sections along the slot of the fan beam. Signals from the equalization detector control corresponding variable attenuation sections in the slit diaphragm.
According to the Vlasbloem et al. patent, the light detection device used for equalization could consist of a series of photosensitive elements on the housing of the scintillator or a series of lenses and photomultiplier tubes. In one alternative embodiment, a single CCD matrix may be used for acquiring a digitized image and for controlling equalization. In another alternative embodiment, a large area, stationary scintillation screen is used in conjunction with an equalization detector comprising vertically arranged, strip-like photoconductors disposed at the front of the screen.
The foregoing prior-art arrangements have serious drawbacks which have limited the usefulness, efficiency, and cost effectiveness of scanning equalization radiography systems in the clinical environment. Movable equalization detectors require a mechanical linkage between the moving x-ray tube/diaphragm combination and the detector. To reduce sensitivity to scattered radiation, the prior art required a scanning slot located between the body and the imaging detector, the scanning slot being mechanically-coupled or servo-coupled to the slit mechanism controlling the fan beam. Such mechanical linkage is susceptible to breakage and interferes with patient and film cassette positioning. The use of a CCD matrix is expensive and is not readily adaptable to systems using film. The embodiment with a large scintillating screen and strip-like photoconductors is susceptible to signal degradation from scatter transverse to the fan beam unless the strips are located close to the screen. All of the prior-art arrangements have equalization detector channels of fixed size and so are unable to properly control systems with variable source-to-image distances (SIDs) which cause the image area covered by each variable attenuation section of the slit diaphragm to vary.
Accordingly, it is a principal object of the present invention to provide scanning equalization without the above-mentioned drawbacks.
It is another object of the invention to provide a scanning equalization method and apparatus employing a stationary detector which avoids signal contamination by scatter.
It is a further object of the invention to provide a scanning equalization method and apparatus employing a stationary detector adaptable to variable source-to-image distance.
It is yet another object to provide means to convert existing radiography systems to perform scanning equalization with minimum modification to the existing system and to avoid introducing moving mechanical components at the x-ray receptor.